[masraum]

I didn't click the link, but yes, the underground "cities" in Turkey are amazing. One, Derinkuyu, is something like 14 levels and 85m deep and could hold 20k people. Another is 10 levels and 40m deep and could house 60k people for 3 months.
I think I've read that the stone that they had to dig through was relatively soft volcanic stone, and these folks were in a prime location that required extraordinary measures to stay safe from groups coming from the east (Persians, Mongols, etc...) and from the west (Greeks, Romans, etc...). And these are said to have been used off and one for hundreds of years by various groups which gives plenty of time for expansion into what we know of today.